Recent Patents on Chemical Engineering 2008, 1, 27-40
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1874-4788/08 $100.00+.00
吸血情圣© 2008 Bentham Science Publishers Ltd.
Surface Chemical Functional Groups Modification of Porous Carbon
Wenzhong Shen*,1, Zhijie Li 2 and Yihong Liu 1
1
State Key Laboratory of Heavy Oil, China University of Petroleum, Dongying, Shandong, 257061, P. R. China
2
Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu, Sichuan, 610054, P. R. China
Received: August 29, 2007; Accepted: September 11, 2007; Revid: November 2, 2007
asiantube8
Abstract: The surface chemistry and pore structure of porous carbons determine its application. The surface chemistry could be modified by various methods, such as, acid treatment, oxidization, ammonization, plasma, microwave treatment, and so on. In this paper, the modification methods were illustrated and compared, some new methods also reviewed. The surface chemical functional groups were determined by the treatment methods, the amminization could increa its basic property while the oxidization commonly improved its acids. In the end, the commonly characterization methods were also mentioned. Some interesting patents are also discusd in this article.
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Keywords: Porous carbon, surface chemical groups, modification, characterization. 1. INTRODUCTION
Porous carbons had been widely ud as adsorbents, catalyst/catalyst supports, electronic material and energy storage material due to its higher surface area and larger pore volume.
The specific surface area, pore structure and surface chemical functional groups of porous carbon determined its applications [1-2]. The pore structure of porous carbon could be controlled by various
routes, such as, activation conditions (activation agent, temperature and time), precursor, templates, etc. The surface chemical functional groups mainly derived from activation process, precursor, heat treatment and post chemical treatment.
The surface functional groups anchored on/within carbons were found to be responsible for the variety in physicochemical and catalytic properties of the matters considered [3-5]. So, many rearchers focud on how to modify as well as to characterize the surface functional groups of carbon materials in order to improve or extend their practical applications [5-7]. Ljubisa R. Radovic reviewed the carbon materials as adsorbents in aqueous solution and pointed out that the control of chemical and physical conditions might be harnesd to produce carbon surfaces suitable for particular adsorption applications [8]. Carlos Moreno-Castilla compared the surface chemistry of the carbon has a great influence on both electrostatic and non-electrostatic interactions, and can be considered the main factor in the adsorption mechanism from dilute aqueous solutions [9].
Modification of the surface chemistry of porous carbons might be a viable attractive route toward novel applications of the materials. A modified activated carbon containing
*Address correspondence to this author at the State Key Laboratory of Heavy Oil, China University o
f Petroleum, Dongying, Shandong, 257061, P. R. China; Tel: +86-546-8395341; Fax: +86-546-8395395; E-mail: different functional groups could be ud for technological applications such as extracting metallic cations from aqueous and nonaqueous solutions, in catalysis, for treatment of waste and toxic effluents produced by a variety of chemical process, and so on.
The heteroatoms on the surface of activated carbon took significant role on its application. The heteroatoms of porous carbon surface mainly contained oxygen, nitrogen, hydrogen, halogen, etc, which bonded to the edges of the carbon layers and governed the surface chemistry of activated carbon [10]. Among the heteroatoms, the oxygen-containing functional groups (also denoted as surface oxides) were the widely recognized and the most common species formed on the surface of carbons, which significantly influenced their performance in nsors [11], energy storage and conversion systems [12-14], catalytic reactions [15], and adsorptions [16-18]. The surface oxygen-containing functional groups could be introduced by mechanical [19, 20], chemical [21, 22], and electrochemical routes [23]. The employment of oxidizing agents in wet or dry methods was reported to generate three types of oxygen-containing groups: acidic, basic, and neutral [24-27]. Bad on the above modifications, a continuous supply of suitable oxidizing agents into the pores of a carbon matr
ix was believed to be a key factor determining the successful introduction of reliable oxygen-containing functional groups onto the surface of carbon materials.
In addition, the nitrogen-containing groups generally provide basic property, which could enhance the interaction between porous carbon and acid molecules, such as, dipole-dipole, H-bonding, covalent bonding, and so on. The nitrogen groups were introduced by ammine treatment, nitric acid treatment and some containing nitrogen molecule reaction.
In this review, we focud on the introducing oxygen and nitrogen heteroatoms on traditional porous carbon (activated carbon and activated carbon fiber) by various methods; the improved application property of modified porous carbon
28 Recent Patents on Chemical Engineering, 2008, Vol. 1, No. 1 Shen et al. was also illustrated. In the end, the ordinarily character-
rization means of oxygen and nitrogen groups were listed.
2. METHODS FOR SURFACE MODIFICATION
The nature and concentration of surface functional
groups might be modified by suitable thermal or chemical
post-treatments. Oxidation in the gas or liquid pha could
be ud to increa the concentration of surface oxygen
groups; while heating under inert atmosphere might be ud
to lectively remove some of the functions. It was shown
that gas pha oxidation of the carbon mainly incread the
concentration of hydroxyl and carbonyl surface groups,
while oxidations in the liquid pha incread especially the
concentration of carboxylic acids [2]. Carboxyl, carbonyl,
phenol, quinone and lactone groups on carbon surfaces were
shown in Fig. (1) [28].
While, the ammonization could introduce the basic
groups, such as, C-H, C=N groups, amino, cyclic amides,
nitrile groups, pyrrole-like structure [29]; which were shown
in Fig. (2) [30]. In addition, the halogen-containing groups
could produce through porous carbon reacted with halogen at
moderate temperature, this modified porous carbon showed
potential application in electrochemistry or batteries [31].
2.1. Acid Treatment
Acid treatment was generally ud to oxidize the porous
carbon surface; it enhanced the acidic property, removed the
mineral elements and improved the hydrophilic of surface.
The acid ud in this ca should be oxidization in nature;
the nitric acid and sulfuric acid were the most lected.
Liu et al. reported that coconut-bad activated carbon
was modified by nitric acid and sodium hydroxide; it showed
excellent adsorption performance for Cr (VI) [32].
Modification caud specific surface area to decrea and the
total number of surface oxygen acidic
groups to increa. Nitric acid oxidization produced positive
acid groups, and subquently sodium hydroxide treatment
replaced H+ of surface acid groups by Na+, and the acidity of
activated carbon decread. The adsorption capacity of Cr
(VI) was incread from 7.61mg/g to 13.88mg/g due to the
prence of more oxygen surface acidic groups and suitable
surface acidity after modification.
Shim et al. also modified the pitch-bad activated
carbon fibers with nitric acid and sodium hydroxide [6]. The
specific surface area of the activated carbon fibers decread
after oxidation with 1 M nitric acid, but the total acidity
incread three times compared to the untreated activated
carbon fibers, resulting in an improved ion-exchange
capacity of the activated carbon fibers. The points of zero
charge of the activated carbon fibers that affect the
lectivity for the ionic species changed from pH 6 to pH 4
by 1 M nitric acid and to pH 10 by 1 M sodium hydroxide
treatment. The carboxyl acid and quinine groups were
introduced after nitric acid oxidation. The carboxyl groups of
activated carbon fibers decread, while the lactone and
ketone groups incread after the sodium hydroxide
treatment. The adsorption capacity of copper and nickel ion
is mainly influenced by the lactone groups on the carbon
surface, pH and by the total acidic groups.
Coal-bad activated carbons were modified by chemical
treatment with nitric acid and thermal treatment under
nitrogen flow [33]. The treatment with nitric acid caud the
introduction of a significant number of oxygenated acidic
surface groups onto the carbon surface, while the heat
treatment increas the basicity of carbon. The pore
characteristics were not significantly changed after the Fig. (1). Simplified schematic of some acidic surface groups bonded to aromatic rings on AC [28].
Fig. (2). The nitrogen functional forms possibly prent in carbonaceous materials [30].
H H
O-
Pyrrole Pyridine Pyridinium Pyridone Pyridine-N-oxide
H
Carboxyl Quinone Hydroxyl
Carbonyl Carboxylic anhyride Lactone
Surface Chemical Modification of Porous Carbon Recent Patents on Chemical Engineering, 2008, Vol. 1, No. 1 29
modifications. The dispersive interactions are the most important factor in this adsorption process. Activated carbon with low oxygenated acidic surface groups has the best adsorption capacity for benzene and toluene.
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The coconut-bad activated carbon was pretreated with different concentrations of nitric acid (from 0.5 to 67%) and was lected as palladium catalyst support [34], the result showed that the amount of oxygen-containing groups and the total acidity on the activated carbons, the Pd particle size and c
atalytic activity of Pd/C catalysts are highly dependent upon the nitric acid concentration ud in the pretreatment. The pretreatment of activated carbon with a low concentration of nitric acid could increa the structure parameters due to removal of the impurities, would be beneficial to create an appropriate density of total acidity environment, and would further improve the Pd dispersion and the catalytic activity of Pd/C catalysts. Meanwhile, a too-large amount of oxygen-containing groups asmbling denly on the activated carbon could influence the Pd dispersion on the activated carbon well.
Peach stone shells were pretreated by H3PO4 and pyrolysis at 500o C for 2 h, then, it was prepared by changing the gas atmosphere during thermal treatment (no external gas, flowing of nitrogen, carbon dioxide, steam or air [35]. High uptake of p-nitrophenol appears, affected to low extent with gaous atmosphere except steam which rais adsorption considerably. Flowing air was the most effective in enhancing the adsorption of methylene blue, which was attributed to the formation of oxygen-functionalities with acidic nature, and to enhancement of wider microporosity. The removal of lead ions was considerably enhanced by running air during thermal treatment (two-fold increa) due to the formation of acidic oxygen-functionalities associated with metal exchange by the negatively charged carbon surface. Li describes the method of eliminating residual carbon from flow able oxide [36].
twitter是什么The activated carbon derived from poly(VDC/MA) was treated with HNO3/H2SO4 solutions and heat-treatment in Ar [37]. Acid-treatment incread the adsorption of methyl mercaptan compared with the original activated carbon, and the adsorbed amounts incread with ratio of H2SO4 in HNO3/H2SO4 solutions. Hydrogen bonding between acidic groups formed by acid-treatment and thiol-groups methyl mercaptan played a role in adsorption of methyl mercaptan on activated carbon. Hanberg et al. shows a process and catalyst blend for lectively producing mercaptans and sulfides from alcohols [38].
Surface modification of a coal-bad activated carbon was performed using thermal and chemical methods [39]. Nitric acid oxidation of the conventional sample produced samples with weakly acidic functional groups. There was a significant loss in microporosity of the oxidized samples which was caud by humic substances that were formed as a by-product during the oxidation process. However, thermal treatment produced a carbon with some basic character while amination of the thermally treated carbon gave a sample containing some amino (-NH2) groups.
The formation of the weakly acidic functional groups on porous carbon surface were thought to be similar to the reaction involving the oxidation of 9,10-dihydrophen-anthrene and diphenylmethane with nitric acid [40], and the mechanism was displayed in Fig. (3). The formation of the dicarboxylic g
roup was thought to occur on the aliphatic side of the molecule especially if the side chains consisted of more than one carbon atom (reaction (a)). The reaction was initiated by the splitting of the C-C at the a-position of the benzylic carbon atom. Oxidation involving a methylene (-CH2-) group would result in the formation of a ketone as shown in reaction (b). Nitrogen could be added to the carbon by a similar reaction as in the nitration of benzene. The mechanism would involve the formation of the highly reactive nitronium ion (NO2-), which would ultimately form the nitrated product as shown in reaction (c).
The amination reaction was achieved via a two stage process. The first stage was the nitration stage where the nitric acid was mixed with concentrated sulphuric acid to form the nitronium ions which then reacted via electrophilic substitution of the hydrogen ion of the carbon matrix as shown in reaction (d). The formed nitro-species formed was reduced using a suitable reducing agent and in this ca sodium dithionite was employed. This result then showed the effectiveness of the reduction reaction shown in reaction (e). This modification process was another example of the application of a classic organic reaction on activated carbon modification. The reaction was shown in the illustration of the amination of phenanthrene.
Calvo et al. reported that the surface chemistry of commercial activated carbon was one of the factor
s determining the metallic dispersion and the resistance to sintering, being relevant the role of surface oxygen groups [41]. The surface oxygen groups were considered to act as anchoring sites that interacted with metallic precursors and metals increasing the dispersion, with CO-evolving complexes significantly implied in this effect. On the other hand, CO2-evolving complexes, mainly carboxylic groups, emed to decrea the hydrophobicity of the support improving the accessibility of the metal precursor during the impregnation step. The treatment of activated carbons with nitric acid led to a higher content in oxygen surface groups, whereas the porous structure was only slightly modified. As a result of oxidation, the dispersion of Pd on the surface of activated carbon was improved.
Santiago
et al. compared veral activated carbons for the catalytic wet air oxidation of phenol solutions [42]. Two commercial activated carbons were modified by HNO3, (NH4)2S2O8, or H2O2 and by demineralisation with HCl. The treatments incread the acidic sites, mostly creating lactones and carboxyls though some phenolic and carbonyl groups were also generated. Characterisation of the ud activated carbon evidenced that chemisorbed phenolic polymers formed through oxidative coupling and oxygen radicals played a major role in the catalytic wet air oxidation over activated carb
on.
Also, citric acid was ud to modify a commercially available activated carbon to improve copper ion adsorption from aqueous solutions [25]. It was found that the surface modification by citric acid reduced the specific surface area by 34% and point of zero charge (pH) of the carbon by 0.5 units. But the modification did not change both external diffusion and intraparticle diffusion.
30 Recent Patents on Chemical Engineering, 2008, Vol. 1, No. 1 Shen et al.
2.2. Ammonia Treatment
It was well known that nitrogen-containing surface
福布论坛groups gave to activated carbons incread ability to adsorb acidic gas [43]. Practically, nitrogen was introduced into
structure of activated carbon according to veral procedures
including treatment with ammonia or preparation of the
adsorbent from nitrogen-containing polymers (Acrylic
textile, polyaryamide or Nomex aramid fibers) [44-46].
Heating of phenol-formaldehyde-bad activated carbon fiber in the atmosphere of dry ammonia at veral
日射temperatures ranged from 500o C to 800o C resulted in a
formation of new nitrogen-containing groups in the structure
of the fiber including C-N and C=N groups, cyclic amides,
nitrile groups (C N) [47], and pyrrole-like surface structures with N-H groups [48]. Despite the changes in the surface chemistry, an outcome of heating of activated
carbons in ammonia atmosphere might also be changed in
porosity of the treated carbon. As it reported, extensive heat-
treatment with gaous ammonia might cau changes in the
relative amounts of macropore, mesopore and micropores of
commercial activated carbon [42].
In any ca, since introducing of nitrogen-containing
surface groups made activated carbon more alkaline and so
speedincread adsorption of acidic agents is expected.
The commercial activated carbons were treated by gaous NH3 ranging from 400o C to 800o C for 2 h [49]. The CH and CN groups appeared after NH3 treatment. It demonstrated enhanced adsorption of phenol from water due to the formation of nitrogen-containing groups during ammonia-treated, which could form hydrogen bond with phenol.
A ries of activated carbon fibers were produced by treatment with ammonia to yield a basic surface [47]. The adsorption isotherms of an acidic gas (HCl) showed a great improvement in capacity over an untreated acidic fiber. The adsorption was completely reversible and therefore involved the enhanced physical adsorption instead of chemisorption. This demonstrated that activated carbon fibers could be tailored to lectively remove a specific contaminant (acidic gas) bad on the chemical modification of their pore surfaces.
Commercial activated carbon and activated carbon fiber were modified by high temperature helium or ammonia treatment, or iron impregnation followed by high temperature ammonia treatment [50]. Iron-impregnated and ammonia-treated activated carbons showed significantly higher dissolved organic matter uptakes than the virgin activated carbon. The enhanced dissolved organic matter uptake by iron-impregnated was due to the prence of iron species on the carbon surface. The higher uptake of ammonia treated was attributed to the enlarged carbon pores and basic surface created during ammonia treatment.
A commercial raw granular activated carbon was modified by polyaniline to improve arnate adsorption [51].
Fig. (3). The formation of acidic functional groups by nitric acid and amination reaction by thermal treatment [38].
5HNO3
2HNO3
HNO3
HNO3
NH3
NO2
2HNO2
highsociety+
thunderbirds
+
+
+
+
H2O
24
+
OH
H2O
H
H
(a)
(b)
(c)
(d)
(e)
224
O
OH
O
+
+
Surface Chemical Modification of Porous Carbon Recent Patents on Chemical Engineering, 2008, Vol. 1, No. 1 31
It was found that the modification did not change the specific surface area. The content of the aromatic ring structures and nitrogen-containing functional groups on the modified granular activated carbon was incread. The surface positive charge density was dramatically incread in acidic solu
tions. The prence of humic acid did not have a great impact on the arnic adsorption dynamics. The modification significantly enhanced the adsorption of humic acid onto the carbon. Meanwhile, the arnate was reduced to arnite during the process.
Lin et al. provided a method for minute deposition of polyaniline onto microporous activated carbon fabric could enhance the capacitance of the carbon rving as electrodes for electrochemical capacitors [52]. The result demonstrated that a capacitance enhancement of 50% in comparison with bare carbon could be achieved with minute polyaniline deposition (5wt%) using the deposition method, while only 22% was reached using the conventional method.
2.3. Heat Treatment
The nature and concentration of surface functional groups might be modified by suitable thermal or chemical post-treatments. Heating oxidation in the gas or liquid pha could be ud to increa the concentration of surface oxygen groups, while heating under inert atmosphere might be ud to lectively remove some of the functions. Thermal treatments had been ud to produce activated carbons with basic character and such carbons were effective in the treatment of some organic hydrocarbons [53].
Heat treatment of carbon in an inert atmosphere or under inert atmospheres (hydrogen, nitrogen or argon) flow could increa carbon hydrophobicity by removing hydrophilic surface functionalities, particularly various acidic groups [54-57]. It had been shown that H2 was more effective than inert atmospheres becau it could also effectively stabilize the carbon surface by deactivation of active sites (i.e., forming stable C-H bonds and/or gasification of unstable and reactive carbon atoms) found at the edges of the crystallites. H2 treatment at 900o C could produce highly stably and basic carbons [52, 55], and the prence of a platinum catalyst could considerably lower the treatment temperature [56]. H2-treated carbons were expected to demonstrate much lower reactivity toward oxygen or chemical agents compared to carbons that were heat-treated in an inert atmosphere. The hydrophobic porous carbon effectively removed the non-polar organic molecules from aqueous solution. However, in order to prepare hydrophobic porous carbon, it needed high temperature and inert/reductive atmospheres to remove the heteroatoms on the surface of porous carbon.
The wood, coal-bad activated carbons and a commer-cial activated carbon fiber with different physicochemical characteristics were subjected to heat treatment at 900o C under vacuum or hydrogen flow [58]. Oxygen sorption experiments showed lower amounts of oxygen uptake by the H2-treated than by the vacuum-treated carbons, indicating that H2 treatment effectively stabilized th
e surfaces of various carbons. At low pressures, from 0.001 mmHg to 5 mmHg, adsorption of oxygen was governed by irreversible chemisorption, which was well described by the Langmuir equation. At higher pressures oxygen uptake occurred as a result of physisorption, which was in agreement with Henry’s law. Kinetic studies showed that oxygen chemisorp-tion was affected by both carbon surface chemistry and porosity. The results indicated that oxygen chemisorption initially started in the mesopore region from the high energetic sites without any mass transfer limitation; thus a constant oxygen uptake rate was obrved. Once the majo-rity of the sites were utilized, chemisorption proceeded toward the less energetic sites in mesopores as well as all the sites located in micropores. As a result, an exponential decrea in the oxygen uptake rate was obrved.
Different precursors resulted in various elemental compositions and impod diver influence upon surface functionalities after heat treatment. The surface of heat-treated activated carbon fibers became more graphitic and hydrophobic. Polyacrylonitrile- and rayon-bad activated carbon fibers subjected to heat treatment [59]. The prence of nitride-like species, aromatic nitrogen-imines, or chemi-sorbed nitrogen oxides was found to be of great advantage to adsorption of water vapor or benzene, but the pyridine-N was not. Unstable complexes on the surface would hinder the fibers from adsorption of carbon tetrachloride. The ri in total ash content or hydrogen composition was of benefit to the access of water vapor.
2.4. Microwave Treatment
The main advantage of using microwave heating was that the treatment time could be considerably reduced, which in many cas reprented a reduction in the energy con-sumption. It was reported that microwave energy was derived from electrical energy with a conversion efficiency of approximately 50% for 2450 MHz and 85% for 915 MHz [60].
Thermal treatment of polyacrylnitrile activated carbon fibers had been carried out using a microwave device [61]. Microwave treatment affected the porosity of the activated carbon fibers, causing a reduction in micropore volume and micropore size. Moreover, the microwave treatment was a very effective method for modifying the surface chemistry of the activated carbon fibers with the production of pyrone groups. As a result very basic carbons, with points of zero charge approximately equal to 11, were obtained.
Microwave heating offered apparent advantages for activated carbon regeneration, including rapid and preci temperature control, small space requirements and greater efficiency in intermittent u [62]. Quan et al. investigated the adsorption property of acid orange 7 by microwave regeneration coconut-bad activated carbons[63]. It was found that after veral adsorption-microwave regenera
tion cycles, the adsorption rates and capacities of granular activated carbons could maintain relatively high levels, even higher than tho of virgin Granular activated carbons. The improvement of granular activated carbons adsorption properties resulted from the modification of pore size distribution and surface chemistry by microwave irradiation.
2.5. Ozone Treatment
Ozone as a strong oxidization agent was widely applied in organic degradation; it could also oxidize the carbon material surface to introduce oxygen-containing groups. The
